1. Field of the Invention
This invention relates to cables, more particularly, to a method for producing lengthwise waterproof cables.
2. Description of the Prior Art
"Cables" in the sense of the invention can be electrical and optical cables. Accordingly, the transmitting elements located in the core are electrical conductors or optical fibers. Sealing material (water-blocking material) may be introduced into the hollow spaces or voids within a cable to prevent humidity, particularly water, from penetrating the core of the cable in the event of damage to the cable jacket. This is of special significance in communication cables with plastic-insulated conductors, for example, in which water that has penetrated the cable can easily propagate in the lengthwise direction. If such water propagation is not prevented, the water could cause a short-circuit between the individual transmission circuits in joining areas of two cable lengths. Furthermore, the presence of water in the core of a cable totally deteriorates the cable transmission properties. For that reason, methods and devices have been developed for sealing the cores of cables. For example, a sealing material may be used to impregnate the core of cable to waterproof the cable. The sealing material is provided in the hollow spaces in the core and between the core and jacket. The sealing material may be a petroleum-type mass, called "petrolatum" which is highly viscous at room temperature, and can be liquefied by adding heat.
A method for making a cable containing a sealing material is known from German Document No. DE-PS 27 22 755, for instance. The core of a cable is passed through a pressure chamber containing the sealing material. The sealing material is pressed into and around the core before the jacket is applied. With this method, the pressure in the pressure chamber is kept constant by means of a regulator, to ensure that sufficient petrolatum reaches the core of the cable. A measuring device is installed to measure the pressure in the pressure chamber. The output of the measuring device is supplied to the regulator. The regulator controls the rpm of an electric motor driving a feed pump which is used to pump the sealing material into the pressure chamber. In spite of this pressure control, it is unavoidable that part of the sealing material escapes from the pressure chamber. In particular, it is carried out by the core itself and exits through the pressure chamber opening from which the core emerges.